Hematopoietic stem cell transplant (HCT) has the potential to cure leukemia and other malignant and non- malignant diseases. However, these processes are associated with significant morbidity, and treatment-related mortality (TRM) ranges from 10-30%. This is largely due to graft-versus-host disease (GVHD), in which the donor immune system attacks host tissues. GVHD has been linked to disruptions in the bacteria living in the gut (the microbiota), which in turn may be caused by factors such as conditioning chemotherapy, radiation and antibiotic treatment. GVHD and mortality are mediated by the immune system, whose activity in turn is shaped by gut microbiota. Prebiotics are nondigestible fibers that can be safely given to HCT patients and metabolized by existing bacteria in the gut to positively impact bacterial communities and metabolic products such as butyrate. Butyrate is a short chain fatty acid that is an important energy source for colonic epithelial cells and can promote regulatory T cell development, which in turn can modulate GVHD. Our preliminary data suggest that the administration of the prebiotic galacto-oligosaccharide (GOS) to mice undergoing HCT, can both increase butyrate and decrease GVHD. Here we propose the following aims: Aim 1. Evaluate the safety, tolerability, and efficacy of GOS in a phase 1 and randomized phase 2 trial. Aim 1A: We will determine the maximum tolerated dose (MTD) of GOS in a phase 1 dose escalation study (Years 1-2). We hypothesize that GOS is tolerable in doses that increase bacterial diversity and butyrate levels. Aim 1B: We will evaluate the impact of GOS on HCT outcomes in a randomized placebo-controlled phase 2 trial. (Years 2-5). We hypothesize that patients receiving GOS will have better outcomes including decreased grade II-IV GVHD. Aim 1C: We will compare the pre-HCT microbiome of GOS responders (those who received GOS in the phase 1 and phase 2 trials and had increased bacterial diversity) and non-responders (those with no change or decreased bacterial diversity) to determine elements of the pre-HCT microbiome that may be associated with GOS response. Aim 2. Evaluate the impact of GOS on biomarkers of intestinal and systemic inflammation and GVHD as potential mediators of HCT outcomes. The regenerating islet-derived protein 3 alpha (REG3A), is of particular interest. REG3A is an antimicrobial peptide that may be affected by the microbiota and has been reported as a biomarker of GVHD. We hypothesize that patients treated with GOS will have positive changes in biomarkers of inflammation and GVHD (increased REG3A, primary endpoint). As an exploratory aim, biomarkers will be studied for their associated with prebiotic interventions, changes in the microbiome, bacterial metabolites, immune reconstitution, and HCT clinical outcomes (GVHD, TRM, OS). Our long-term goal is to understand how prebiotic interventions impact the microbiota and bacterial metabolites and use prebiotics to decrease inflammation and improve HCT outcomes. We expect that the data generated from this study will lead to future clinical trials, including a multicenter randomized phase 3 trials, or to microbiota-based diagnostic assays to personalize therapies.